1. Technical Field
The invention relates to a method in accordance with the preamble of claim 1 and to a corresponding apparatus.
2. Description of Related Art
The drives in question are used in industrial application in machine tools, robots, conveying plants, pumps and the like. Since there is potentially a risk for the life and health of the operator from these drives or from these machines in many applications, the drives must be secured. The measures for securing are subject to strict international and European standard provisions with respect to failure safety, reliability and the like. Specifically for electrical power drives, the standard EN 61800-5-2 “Adjustable speed electrical power drive systems; Part 5-2: Safety requirements—Functional” describes a number of drive monitoring functions with whose aid movements causing danger should be monitored and brought to a standstill in a failsafe manner. A number of other standards are likewise relevant such as the standard EN 954 “Safety related parts of control systems” and its successor standard EN 13849. In the following, the terms “failsafe” and “safe” will be used as synonyms in the meaning that the systems and activities labeled thereby are “safe” or can be carried out “safely” in accordance with the standard provisions and faults or malfunctions which could result in a danger are recognized or result in a reaction for danger reduction, as the standards require.
To satisfy the safety demands on drives, external sensors such as encoders were used as well as further sensors for determining the position, speed and torque which in turn have to satisfy different safety standards such as EN/IEC 61946 “Safety of machinery-Electro-sensitive protective equipment (ESPE)” or EN 61508 “Functional safety of electrical/electronic, programmable electronic safety-related systems”.
The use of external sensors is disadvantageous since, in addition to the machine, the sensors actually have to be provided and a cabling corresponding to the safety demands has to be drawn between the switch cabinet in which the control for the machine is located and the field plane. Depending on the structure of the sensor, it can furthermore also be necessary that an additional, redundant sensor has to be installed to achieve the required fault safety of the machine. The effort and so the costs are increased by the assembly of the sensor and the cabling. The sensors which are located in the harsh field plane are at greater risk of failure due to the environmental conditions, whereby the availability of the machine is impaired.
For this reason a device was proposed in DE 10 2005 010 854 A1 to be able to dispense with the additional sensors. In accordance with the teaching of this document, a drive monitor is provided with which the speed, the torque and the direction of rotation can be determined and indeed from the current and voltage profiles such as are provided by a three-phase electronic control system to the electric motor. For this purpose, the drive monitor carries out a vector calculation of voltage U and current I and determines the speed, direction of rotation and torque of the motor from the vectorial data, in particular from the angle of the rotary electric field.
The required vector calculation is very complex and requires a large calculation capacity. The greatest restriction comprises the fact that this drive monitor is only suitable for synchronous motors and not for asynchronous motors since the determination of the speed and the direction of rotation only takes place via the angle of the rotary electric field, but the electric and dynamic properties of the asynchronous motor are neglected.